The present invention relates to an ultrasonic motor and a CRT display device using the ultrasonic motor. More specifically, the present invention relates to an electrostrictive revolution type ultrasonic motor having an improved rotor structure.
The present invention further relates to a driving device for an ultrasonic motor which is required to be driven under a predetermined frequency such as under a resonant condition of the ultrasonic motor as a load.
An ultrasonic motor is utilized as a servo motor in a precise adjustment mechanism because of the ability which permits a very fine operation without necessitating interposing of such as a reduction gear and of a large holding torque during no voltage application. In particular, a so called electrostrictive revolution type ultrasonic motor, which makes use of a disk shaped ceramic piezoelectric stator of which center of gravity precessions through excitation by ultrasonic input power as disclosed in JP-A-63-257474 and JP-A-63-181676 which partly corresponds to U.S. Pat. No. 4,868,446, is suitable for reducing the size and cost thereof because of its comparatively simple structure thereof.
FIGS. 9A and 9B show a structure of a conventional electrostrictive revolution type ultrasonic motor. The ultrasonic motor as illustrated in FIGS. 9A and 9B is constituted by a disk shaped stator 1 formed by a piezo-electric element of ferroelectric substance subjected to a polarization treatment in advance such as PZT, a rotor 2 formed from a phosphor bronze plate in Petri dish shape through drawing and fitted around the circumference of the stator 1, an insulator spacer 6 of Mylar for preventing short circuiting between electrodes formed on the surface of the stator 1 and the metallic rotor 2 and others. The rotor 2 is provided with three projecting portions 3 formed by drawing and the rotor 2 contacts the outer circumference of the stator 1 via these projecting portions 3 with a proper contacting pressure.
For divided fan shaped electrodes 11 through 14 are formed by spattering on both faces of the stator 1 and one of the electrodes on the front face thereof is connected to the electrode on the back face thereof facing in 180.degree. relation each other so as to constitute two sets of electrodes corresponding to two phases. Further, the two sets of the electrodes are constituted to receive two phase pulse like voltage of which frequency is tuned to the resonant frequency via a coil spring (not shown) so as not to disturbe the mechanical resonance of the stator 1. The details for driving the stator are disclosed in the above indicated patent documents, therefore the explanation thereof is omitted in the present specification. In the ultrasonic motor as illustrated in FIGS. 9A and 9B having the above explained structure, one electrode portion of the stator 1 slightly expands, normally below 1.mu.m, in response to the polarity of the applied pulse like voltage, and the other portion displaced by 180.degree. with respect to the one electrode portion contracts, as a result, the center of gravity of the stator 1 moves eccentrically thereby to cause a so called precession. Such motion is transferred to the projecting portions 3 of the rotor 2 via frictional contact and induces a rotating torque in the rotor 2 to rotate the same.
However, with the conventional ultrasonic motor having the above explained structure, the generated rotating torque is small, variation of the generated rotating torque is large by product by product and a complex and fine adjustment of the spring pressure of the respective projecting portions was indispensable. Further, even when a necessary rotating torque is generated, a time depending torque variation due to such as damaging of the stator contacting face was large, therefore the ultrasonic motor having the above explained structure has not been reduced into practice as a mass-producable product until now. Still further, since metal is used for the rotor material in the conventional ultrasonic motor, such ultrasonic motor is not suitable for an application wherein mutual action or interference with respect to electric field and magnetic field causes problems, for example, an application for a flyback voltage adjustment for focusing adjustment in a CRT display device in which an intense electro-magnetic field is generated or a medical use electronic measurement equipment in which a slight electro-magnetic disturbance causes problems.
According to experimental study performed by the present inventors, it is found out that the above conventional problems are caused by the rotor formed from a phosphor bronze plate through drawing method. The drawing is an only method of forming a thin spring material such as a phosphor bronze plate in mass-production scale, however it is found out that the variation in the dimensional accuracy through such drawing reaches about .+-.50.mu.m, and such variation is too large exceeding an allowable variation of about .+-.15.mu.m for flexing amount when the rotor of the electro-strictive revolution type ultrasonic motor is contacted. Further, it is also found out that hardness change due to stress during spreading by the drawing operation causes variation. Still further, because of a high hardness of the contacting portion of the rotor which is formed from a metallic spring material the stator contacting face is likely to be damaged in time depending manner during the contacting rotation which brings about a large variation of frictional contacting force and a time depending change thereof.
An ultrasonic motor is one examples which can operate only by a predetermined frequency signal called as a resonant frequency signal. Such a machine necessitates a so called frequency tracking operation in which the operating frequency is tuned to the resonant frequency prior to the primary control operation thereof in order to maintain the resonant condition in response to the state change of the machine.
An ultrasonic motor generates a maximum torque at its resonant condition and when the operating frequency deviates from the resonant frequency, the torque generation is either extremely reduced or totally ceased to stop the operation of the motor such that the resonant frequency tracking operation is an indispensable measure so as to ensure a stable operation of the ultrasonic motor even under temperature change and load variation thereof.
Conventional resonant frequency tracking method uses such as a synchronous method using a phase-locked loop (PLL) as disclosed for example in JP-A-62-85684 and a resonant frequency searching method using a digital storage means as disclosed for example in JP-A-63-02774. However, with the former method when noises are high a stable operation can hardly be realized, and further with the later method using the digital technology when a high resolution searching is required the circuit scale is enlarged and complexed as well as the searching time thereby is prolonged.